Ooid Mineralogy and Diagenesis of Upper Mississippian Pitkin Formation, Arkansas
E. Heydari, R. D. Snelling, W.C. Dawson, M. L. Machain
The Pitkin formation is a marine oolitic-bioclastic limestone exhibiting
shoal, lagoonal, beach, and tidal-channel facies. Ooids formed on the Pitkin
carbonate shelf show lateral and vertical
variations in original mineralogy. In
a section south of Mountain View, Arkansas, the lower marine shoal and shoreface
facies are composed of originally aragonitic ooids. These ooids are replaced by
neomorphic calcite and exhibit elevated strontium and low magnesium
concentrations. The overlying lagoonal facies are composed of originally calcite
ooids that have retained their original radial fabrics. These ooids exhibit low
strontium and high magnesium concentrations. Ooids in sections to the west of
Mountain View are all calcitic.
Early diagenesis of the formation is dominated by marine cementation.
Postdepositional diagenesis encompasses extensive dissolution of originally
aragonite and magnesian calcite components, precipitation of a late unzoned
ferroan calcite, and minor amounts of saddle dolomite. Grainstone units lack any
intergranular porosity. Pressure solution
contacts among grains in grainstones
are generally uncommon. Cementation alone is responsible for destruction of all
of the original porosity in these rocks. Intensive pressure
solution
is observed
only in packstone units not affected by early marine cementation.
AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.